Influence of Intercritical Temperature on the Microstructure and Mechanical Properties of 6.5 Pct Ni Steel Processed by Ultra-fast Cooling, Intercritical Quenching and Tempering

A novel treatment comprised of ultra-fast cooling, intercritical quenching and tempering is proposed to modify the microstructure and improve the mechanical properties of low-concentration Ni-containing steel (6.5 pct Ni steel). The influence of intercritical temperature on the microstructure and mechanical properties has been systemically investigated. The results reveal that a mixed microstructure of alloy-depleted intercritical ferrite and alloy-enriched fresh martensite and retained austenite is obtained after intercritical quenching. Moreover, reversed austenite is formed at the grain boundaries of fresh martensite and retained austenite during subsequent tempering. Furthermore, the increase in intercritical temperature promoted the formation of lath-like fresh martensite, leading to the refinement of intercritical ferrite and the formation of secondary acicular reversed austenite. When the intercritical temperature was increased to 670 °C and 700 °C, reversed austenite exhibited sufficient stability at − 196 °C because of the fine-grained structure and acicular morphology. Finally, an optimal combination of strength and cryogenic toughness has been achieved by intercritical quenching from 700 °C. The obtained high yield strength can be ascribed to the decrease in the amount and size of intercritical ferrite, whereas excellent cryogenic toughness can be attributed to the increase in fine-grained stable acicular reversed austenite and decrease in effective grain size.